1. Field of the Invention
The present invention relates to a system and method for controlling a minimum flow rate of a variable geometry turbocharger and, more particularly, to a system and method for controlling a minimum flow rate of a variable geometry turbocharger that can improve the starting performance of a vehicle and prevent the generation of surge noise and smoke remarkably, compared with the convention system in which the position of a stopper is fixed hardwarely, by configuring the system capable of adjusting the position of the stopper to satisfy a target boost pressure under fixed conditions.
2. Description of Related Art
Recently, a variable geometry turbocharger (VGT) has been widely applied to a diesel engine in order to achieve high-output and low-pollution.
The variable geometry turbocharger has been developed for providing high torque and high power and, at the same time, for obtaining a sufficient torque margin at low speed compared with the conventional waste gate turbocharger (WGT) by variably applying the passage area of exhaust gas introduced in a turbine.
In the variable geometry turbocharger as depicted in FIG. 1, in order to maximally ensure the amount of air in a low load region that is insufficient in the conventional waste gate turbocharger, vanes are adjusted to minimize the flow area in a low speed region and maximize the flow area in a high speed region, thus increasing the responsiveness in the low speed and low load region and also reducing the exhaust gas by ensuring a sufficient amount of air.
The variable geometry turbocharger will be described with reference to FIGS. 2 to 6 as follows.
As depicted in the figures, the variable geometry turbocharger comprises a compressor 10, a turbine 11 and a flow regulator 12 disposed therebetween for regulating the flow of exhaust gas.
The flow regulator 12 regulates the angle positions of vanes 16 to improve the flow performance of the exhaust gas and comprises a unison ring 14 established in a housing 13 of the turbine 11, a plurality of vanes 16 and a disk 17, established on one side of the unison ring 14 at regular intervals and moved within the range that they are not in contact with a turbine wheel 15, a bushing 18 and a lever 19 that operate the vanes 16 and the disk 17, and an actuator 21 connected to the lever 19 through a actuator rod 20 and operated by vacuum pressure.
Reference numeral 22 denotes a link, of which one end is supported to the unison ring 14 and the other end is connected to the vane 16 to be operated with the vane 16.
Moreover, a screw (bolt) type stopper 23 restricting the displacement of the actuator rod 20 is provided in the housing 13. Here, the lever 19, connected to a front end of the actuator rod 20 with a pin (not shown), comes in contact with the stopper 23 and the stopper 23 restricts the rotation of the lever 19 and the movement of the actuator rod 20, thus setting a minimum flow rate of the turbocharger.
Like this, if the actuator rod 20 moves back and forth, the disk 17 rotates centering on an axis thereof by the lever 19 and the bushing 18. Accordingly, the angle of the vane 16 can be varied through the link 22, of which one end is supported to the unison ring 14.
That is, the operation angle of the vanes 16 is set most suitably by the actuator 21 using vacuum pressure in the variable geometry turbocharger.
The optimum position of the vanes under the various driving conditions is determined according to map information of ECU; however, the minimum angle of the vanes 16 is determined by the mechanical stopper 23 at an early stage.
As one of the matching items in the development process, the position of the stopper preset by an engine developer is measured by a VGT maker using a master VGT to mass produce the VGTs.
Meanwhile, according to the variable geometry turbocharger as described above, it is possible to increase the amount of intake air by controlling the cross sectional area of a turbine entrance to maximize the energy efficiency without the increase of exhaust gas, thus obtaining higher output power. Such increase in the output power can inhale a greater deal of air in the same load and, thereby, prevent the generation of undesirable components of incomplete combustion such as exhaust fumes (PM) caused by the insufficient air.
Moreover, it is possible to ensure an exhaust gas margin by the reduction of the exhaust fumes (NOx/PM Trade-Off) and increase the excess force of the vehicle by the increased engine power, thus providing better fuel efficiency under the same load conditions.
FIG. 1 shows an example of the position control of vanes according to the driving regions. As depicted in the figure, if the vanes are closed in a low speed region, it is possible to provide an increase in the torque in the low speed region and an improvement in the responsiveness through the increased boost pressure, thus improving the starting performance of the vehicle. Whereas, the vanes are opened in a high speed region to increase the exhaust flow and thereby improves the output power.
The control of the variable geometry turbocharger generally complies with a PID control, and the behavior of the actuator that controls the vanes is determined by the vacuum pressure output through a solenoid valve controlled by regulating the opening and closing duty ratio of PWM, if ECU receives signals from the respective sensors, such as an air flow sensor, a boost pressure sensor, a water temperature sensor, etc., and outputs control signals.
Here, the control signals of ECU are determined by a difference calculated by comparing a target boost pressure with an actual boost pressure detected by the boost pressure sensor based on input values from the various sensors.
A major factor that determines the increase in the torque in the low speed region and the improvement in the responsiveness, which are the major characteristics of the variable geometry turbocharger, is to determine the minimum flow rate when the nozzle area is minimum, that is, to determine the minimum area of the path, through which the exhaust gas passes, as shown in the top left of FIG. 1.
As described above, the rotational movement of the vanes 16 is made when the actuator rod 20, connected to the unison ring 14 rotating the vanes 16, the bushing 18 and the lever 19 in turn, moves back and forth. The minimum area of the vanes 16 is determined through a test by regulating the position of the screw (bolt) type stopper 23 to satisfy a target boost pressure in the low speed (generally, less than 1,000 rpm) full load region.
That is, the minimum flow point of the variable geometry turbocharger is determined by the screw type stopper hardwarely and it is impossible to regulate the minimum flow point by ECU having the PID control algorithm.
The method for setting the minimum flow rate determined through the test sets the front and rear positions of the stopper 23 to obtain a target boost pressure at a point (hereinafter, referred to as a minimum flow region) where the lever 19 is in contact with the stopper 23 and then fixes the stopper 23 using a fixing nut.
The minimum flow rate set like this is a very important factor that affects the vehicle starting performance and the generation of surge noise and smoke but it causes numerous problems in the conventional method since it is restricted hardwarely by the stopper position.
It is advantageous to reduce the minimum flow rate for the improvement of the vehicle starting performance but if adjusted below an optimum flow rate, the surge noise is generated and the harmful exhaust gas such as smoke is increased.
Since the minimum flow region is the minimum area of the vanes, where the actuator lever comes in contact with the stopper hardwarely, the actual boost pressure cannot meet the target boost pressure, even if a signal of a maximum duty is output in case of the PID control.
In the conventional art, the flow range in the minimum flow region has been controlled to be within a specific deviation range through a part test by the variable geometry turbocharger maker; however, even if it is within the deviation range, the minimum flow rate in the actual engine has many deviations due to the limitations in the part test.
Moreover, abrasions in the connection portion between the vanes and the actuator and in the stopper occur with the increase in the driving time to vary the minimum flow rate, thus resulting in various problems of the deterioration of vehicle starting performance (if the minimum flow rate is greater than an optimum value), the generation of surge noise and excessive smoke (if the minimum flow rate is smaller than the optimum value), etc.
FIGS. 7 and 8 are graphs for illustrating the problems in accordance with the conventional art, in which FIG. 7 shows boost pressure characteristic deviations according to minimum flow deviations for an engine and FIG. 8 shows boost pressure characteristic deviations according to minimum flow deviations in a vehicle.
Referring to those figures, since the boost pressure is always insufficient against a target value due to the response delay of the turbocharger in an initial starting phase, the vehicle would be driven under the circumstances in that the PID duty is in the maximum and the vanes are in the minimum cross sectional areas, i.e., at the minimum flow rate.
Here, if the minimum flow rate is excessive, the boost formation is delayed in preparation for an optimum matching state, which may result in the deterioration of vehicle starting performance and the excessive generation of smoke.
Moreover, if the minimum flow rate is too little, the initial boost formation exceeds a target boost pressure, which may result in turbo damage due to the excess of speed endurance limit caused by the over boost.
Here, if the accelerator pedal is suddenly released, while the boost pressure is high, it enters a compressor surge region as the amount of air is suddenly reduced (where it becomes unstable by flow separation and reverse flow due to the lack of air, while the compressor rotational speed is high), thus causing heavy noise.
Since the respective variables of the PID control are optimized for the optimum matching state, it is impossible to overcome the hardware limitations even with the PID control.
Accordingly, it is necessary to solve the problems of the deterioration of vehicle starting performance and the generation of noise, etc., caused by the characteristic variations of the boost pressure formation due to the difference in the boost pressure formations between the part and the actual engine and the abrasions with the passage of driving time.